www.nature.com/scientificreports OPEN Rodent host population dynamics drive zoonotic Lyme Borreliosis and Orthohantavirus infections in humans in Northern Europe Mahdi Aminikhah1*, Jukka T. Forsman2, Esa Koskela3, Tapio Mappes3, Jussi Sane4, Jukka Ollgren4, Sami M. Kivelä1 & Eva R. Kallio3* Zoonotic diseases, caused by pathogens transmitted between other vertebrate animals and humans, pose a major risk to human health. Rodents are important reservoir hosts for many zoonotic pathogens, and rodent population dynamics afect the infection dynamics of rodent-borne diseases, such as diseases caused by hantaviruses. However, the role of rodent population dynamics in determining the infection dynamics of rodent-associated tick-borne diseases, such as Lyme borreliosis (LB), caused by Borrelia burgdorferi sensu lato bacteria, have gained limited attention in Northern Europe, despite the multiannual abundance fuctuations, the so-called vole cycles, that characterise rodent population dynamics in the region. Here, we quantify the associations between rodent abundance and LB human cases and Puumala Orthohantavirus (PUUV) infections by using two time series (25-year and 9-year) in Finland. Both bank vole (Myodes glareolus) abundance as well as LB and PUUV infection incidence in humans showed approximately 3-year cycles. Without vector transmitted PUUV infections followed the bank vole host abundance fuctuations with two-month time lag, whereas tick-transmitted LB was associated with bank vole abundance ca. 12 and 24 months earlier. However, the strength of association between LB incidence and bank vole abundance ca. 12 months before varied over the study years. This study highlights that the human risk to acquire rodent-borne pathogens, as well as rodent-associated tick-borne pathogens is associated with the vole cycles in Northern Fennoscandia, yet with complex time lags. Zoonotic diseases—diseases caused by pathogens transmitted between non-human vertebrate animals and humans—pose a substantial health threat to humans1–3. While zoonotic pathogens contribute to emerging infec- tious diseases, such as the current pandemic of coronavirus disease (COVID-19)4,5, many zoonotic pathogens are persistently transmitted from wildlife hosts to humans 3,6–10. For instance, Orthohantaviruses (genus Orthohan- tavirus, family Bunyaviridae) are rodent-borne pathogens 11 that are transmitted from rodent hosts to humans without vectors, as inhaled aerosols12,13. Orthohantaviruses cause hemorrhagic fever with renal syndrome (HFRS) and hantavirus cardiopulmonary syndrome (HCPS) depending on the virus, with approximately 100,000 diag- nosed infections annually across the world 14. In Europe, the most common rodent-borne disease is nephropathia epidemica (NE), which is a mild form of HFRS, caused by Puumala orthohantavirus (PUUV)15. Meanwhile, many wildlife-originating zoonoses are transmitted by vectors. For example, Borrelia burgdorferi sensu lato (s. l.) bacteria, including rodent-associated Borrelia afzelii, which cause tick-borne Lyme Borreliosis (LB) disease in humans, are transmitted by Ixodes spp. ticks from wildlife reservoir hosts to humans 16. It has been estimated that there have been approximately 230,000 human infections in Western Europe in recent years 17, whereas the Centers for Disease Control and Prevention (CDC) have estimated more than approximately 300,000 new cases in the United States yearly18. In order to predict and reduce the risks caused by wildlife-originated zoonotic dis- eases, understanding the role of their reservoir host dynamics in driving human infection dynamics is crucial 19–21. 1Department of Ecology and Genetics, University of Oulu, PO Box 3000, 90014 Oulu, Finland. 2Natural Resources Institute Finland (Luke), University of Oulu, Paavo Havaksen tie 3, 90014 Oulu, Finland. 3Department of Biological and Environmental Science, University of Jyväskylä, P.O. Box 35, 40014 Jyväskylä, Finland. 4Department of Health Security, National Institute for Health and Welfare, Helsinki, Finland. *email: [email protected]; [email protected] Scientifc Reports | (2021) 11:16128 | https://doi.org/10.1038/s41598-021-95000-y 1 Vol.:(0123456789) www.nature.com/scientificreports/ Rodents (order Rodentia) are important reservoirs for zoonotic pathogens7,22. Ofen, rodent population densities fuctuate either seasonally or annually, which may translate into variations in human infection risk caused by rodent-borne pathogens 19. For instance, Northern Fennoscandian vole cycles, which are multiannual abundance fuctuations of common rodents, three to fve years in length, have a high amplitude, coincide with all sympatric vole species and are synchronous over large areas 23–26, are known to afect human PUUV infection incidence21,27–30. Even though rodents are well-known reservoirs of pathogens, the role of rodent population fuctuations in the epidemiology of rodent-associated tick-borne diseases remains to be clarifed. Mostly food-driven rodent abundance fuctuations have been shown to infuence density of nymphal ticks, B. burgdorferi s. l. infection prevalence in nymphs and/or the density of infected nymphs in the following year in North America31–33 and in Central Europe34–37. Meanwhile, the mostly predator-driven vole abundance fuctuations in Fennoscandia 23–26 have received less attention in driving human tick-borne disease risk. While density of infected nymphs and nymphal infection prevalence are commonly used as the measure for human infection risk31,33,35–37, studies quan- tifying whether rodent abundance variations are translated into human disease cases are more scant. Moreover, the fndings are not unequivocal with some studies showing positive but others showing no association between rodent abundance and following year human disease38–40. Consequently, it remains enigmatic how the circula- tion of rodent-associated tick-borne pathogens is maintained in over very low reservoir host densities that may last over extended time periods, which characterise the multiannually fuctuating vole populations in Northern Europe41. Yet, LB persists in these regions, with approximately 6000–7000 human infections annually, mainly caused by B. afzelii that is transmitted by generalist ticks I. ricinus and I. persulcatus42,43 in Finland44. One of the most common rodent species in Northern Europe is the bank vole (Myodes glareolus)45, which is considered as a hyperreservoir (species that carry out two or more zoonotic pathogens) of zoonotic pathogens 7, including PUUV46 and tick-borne Borrelia afzelii, one of the agents causing LB47,48. An association between bank vole abundance and LB incidence is expected as Borrelia infection prevalence in ticks is positively correlated with bank vole abundance35,49,50. Here, we aim to characterize the temporal dynamics of Lyme Borreliosis and PUUV infections in humans in relation to the abundance fuctuations of the rodent reservoir host, the bank vole, in Finland. While human PUUV infection epidemics has been shown to follow bank vole abundance fuctuations with short time lags21,29,51, we were specifcally interested in quantifying the relationship between human LB cases and rodent abundance fuctuations. Human LB infection incidence is expected to follow bank vole abundance with a much longer time lag than human PUUV infection incidence, as Borrelia bacteria are transmitted from the reservoir hosts to humans by ticks34. Te tick life-cycle necessarily introduces time lags in pathogen transmission; ticks typically acquire the pathogen in the larval stage when feeding on a reservoir host and can then transmit the pathogen to humans in subsequent life stages, either as a nymph (typically in the following year) or as an adult (typically two years later)52,53. Methods Ethics statement. Te study was conducted in accordance with the efective national and institutional regulations and guidelines (currently Finnish Act on the Protection of Animals used for Scientifc or Educational Purposes (497/2013), that follows the directive 2010/63/EU), and was licensed by the National Project Authori- sation Board (ESLH-2008-04660/Ym-23, ESLH-2009-09663/Ym-23, ESAVI-3834-04.10.03/2011, ESAVI-7256- 04.10.07/2014, ESAVI-3981/2018). Te human infection incidence data have not included any personal infor- mation, only numbers of cases per hospital district. Terefore, no ethics permission for the use of the human data was needed. Human infection data. We used National Institute for Health and Welfare data on monthly incidences (infections per 100 000 inhabitants) of laboratory-diagnosed LB (LBlab) caused by Borrelia spp. infections and NE caused by PUUV infections from 1995 to the May of 2019 (data available at www. thl. f/ ttr/ gen/ rpt/ tilas tot. html), as well as data on symptom-based LB (LBsym) cases that are available from 2011 to the May of 2019 (outpatient healthcare visits from the primary healthcare units). LBsym data consist of illnesses diagnosed with the ICD-10 code A69.2 (Lyme borreliosis), which basically refers to the clinical diagnosis of erythema migrans, the ring-like rash. We used human infection data from Northern Savo (Pohjois-Savo, PS) and Central Finland (Keski-Suomi, KS) hospital districts, these hospital districts surrounding the rodent monitoring site in Central Finland (Fig. S1). Bank vole data. Bank vole abundance was monitored in 100 km2 area in Konnevesi in Central Finland (62° 34′ N, 26° 24′ E) 1995–2019 (Fig. S1). Trapping was conducted four times per year, in May, June/July, August and October/November in